Nipped rolls are used in a vast number of continuous process industries including papermaking, steel making, plastics calendering and printing. The characteristics of nipped rolls are particularly important in papermaking. In the process of papermaking, many stages are required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock onto paper machine forming fabric or wire. Upon deposition, the white water forming a part of the stock flows through the interstices of the forming fabric, leaving a mixture of water and fiber thereon. The forming fabric, and subsequently the felt, then supports the mixture, leading it through several dewatering stages such that only a fibrous web or matt is left thereon.
One of the stages of dewatering takes place in the press section of the papermaking apparatus. In the press section, two or more cooperating rolls press the fibrous web as it travels on the felt between the rolls. The rolls, in exerting a great force on the felt, cause water to be expressed from the web traveling thereon and the web to become flattened, thereby achieving a damp fibrous matt. The damp matt is then led through several other dewatering stages.
The amount of nip pressure applied to the web and the size of the nip can be important in achieving uniform sheet characteristics. Variations in nip pressure can affect sheet moisture content and sheet properties. Excessive pressure can cause crushing or displacement of fibers as well as holes in the resulting paper product.
Roll deflection, commonly due to sag or nip loading, can be a source of uneven pressure and/or nip width distribution. Worn roll covers may also introduce pressure variations. Rolls have been developed which monitor and alter the roll crown to compensate for such deflection. Such rolls usually have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and provide increased pressure to the floating shell when necessary.
Notwithstanding the problem of roll deflection, the problem of uneven loading across the roll length and in the cross machine direction persists because pressure is often unevenly applied along the roll. For example, if roll loading in a roll is set to 200 pounds per inch, it may actually be 300 pounds per inch at the edges and 100 pounds per inch at the center.
Crown corrections are often made from nip width measurements. For simple crown corrections, the amount of correction may be estimated from:
  C  =            (                        N          E          2                -                  N          C          2                    )        ⁢                            D          1                +                  D          2                            2        ⁢                  D          1                ⁢                  D          2                    where
NE is the nip width at the end of the roll,
NC is the nip width at the center of the roll, and
D1 and D2 are the roll diameters.
This equation is used throughout the paper industry for estimating crown corrections.
One technique for assessing and measuring nip width is discussed in U.S. Pat. No. 6,568,285 to Moore et al., the disclosure of which is hereby incorporated herein in its entirety. This technique involves the use of sensors, typically formed of overlying but non-contacting layers of relatively resistive carbon and highly conductive silver, which are attached to an elongate sheet. The sheet is positioned in the nip. As the sensors extend lengthwise through the nip, the layers of the sensors deform and contact one another, thereby changing the electrical resistivity of the overall sensor. This change in resistivity can be correlated to the contact length of the sensor, which represents the width of the nip. This technique is employed by the NipProfiler® system available from Stowe Woodward LLC, Middletown, Va.
Although this technique has proven to be a reliable manner in which to measure nip width in a variety of locations on a papermaking machine, the device has been unable to provide reliable results in elevated temperature environments (e.g., above 200° F.). As such, it would be desirable to provide an apparatus for sensing nip width that can operate under elevated temperature conditions.